Acanthosis nigricans is a commonly occurring skin lesion which indicates endogenous hyperinsulinemia. We hypothesize that patients with this skin lesion have a variety of hereditary defects in insulin action, all of which are associated with a similar phenotype and a high risk for non-insulin dependent diabetes mellitus. Subjects will have insulin responsiveness evaluated annually, and selected severely affected subjects and their parents will have evaluation of the insulin receptor gene for point mutations.